Migration
Although some scientists define animal migration as any animal movement, this definition becomes cumbersome because it does not distinguish between small-scale daily movements, annual migrations, and irrupting dispersions. Mobile animals tend to move frequently, and migration should be distinct from emigration (directional one-way movement) and dispersal (non-directional one-way movement), and it should refer primarily to regular round trip movement that happens at least once in the life span of the organism. Migration is a spatial behavior pattern that allows animals to locate themselves in the most favorable portions of their habitat for as long as necessary. Such favorable conditions may vary according to season or life history, but in both cases it is related to adaptive fitness. This allows the organism to take in nutrients in excess of energy expenditures and to successfully reproduce.
Generally animal migration can be divided into two areas of study: the behavioral aspects, which concentrates on "how" migration happens; and the ecological aspects, which addresses "why" migrations takes place. The ecological questions also concern evolution, for spatial behavior is an evolved compromise between differing requirements of an organism's life. Most migratory behavior depends on food abundance. In most habitats productivity varies with the seasons, and thus energy availability also varies at all upper levels of food chains. Migration must often accommodate several energy and reproductive requirements. As a result migration patterns tend to be complicated with subsections of migrants taking slightly differing paths at various times to serve different needs.
Many species of North American waterfowl use several breeding areas, from the northern prairies to the Arctic coast, and travel along several major flyways to wintering areas that range from the southern prairies to the estuaries of northern South America. Within that framework, males desert the hens during nesting season and make shorter migrations to molting areas, afterward meeting with the females and newly fledged young during the fall migration. In either of those sites, requirements for habitat differ in terms of water-cover ratios, water permanence, and food preferences.
The altitudinal migration of mountain sheep (Ovis canadensis) in the Rocky Mountains brings them into more productive habitats during the summer in the alpine meadows and into the mountain forest during the winter. Thus, food intake requirements are accommodated and, simultaneously, the sheep take advantage of the microclimate of the forested area during the winter to supplement energy losses from body temperature maintenance.
Migration can also be a response to particular breeding site requirements, mate location, and a combination of several forces acting together. The longest mammal migration, that of the California gray whale (Eschrichtius robustus), places the breeding-ready adults and newly impregnated females in the productive shallows of the Bering and Chukchi Seas, the non-breeding animals in the more patchy feeding grounds of the northeast Pacific Coast, and the calving females in the warm shallow lagoons of the Baja Peninsula.
Resources
Periodicals
Clark, C. W. "Moving With the Heard (Hydrophonic Monitoring of Migratory Bowhead Whales)." Natural History (March 1991): 38–42.
Dybas, C. L. "Secret Creatures of the Nigh; When the Moon is New and Darkness Falls, American Eels Begin Their Eerie Autumn Migration." National Wildlife 28 (October-November 1990): 18–23.
Hansson, L. "The Lemming Phenomenon: Or Why the Legendary Mass Migrations of Rodents Are Restricted to the Extreme North." Natural History (December 1989): 38–43.
Pallace, D. R. "Avian Nations: The Patterns and Problems of Migrating Birds." Wilderness 54 (Fall 1990): 42–9+.
"Recent Developments in the Study of Animal Migration." [Symposium on Recent Developments in the Study of Animal Migration.] American Zoologist 31 (1991): 151–276.
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